A switching arrangement of the kind referred to above can be used generally for fluid, electric or pneumatic switching purposes. In an electric monitoring arrangement disclosed in German published patent application DE-OS 2,532,081, a switching arrangement is provided wherein it is intended to protect a busbar against undesired heating as a consequence of increased current output. For this purpose, a temperature sensor in the form of a bimetal switch is applied from the outside to the busbar. The bimetal switch is connected into the secondary circuit of a transformer having a primary circuit which is connected to a voltage source. A relay is connected in parallel to the bimetal switching circuit as a monitoring arrangement which is picked up with the voltage present and maintains the current supply for the busbar to be monitored by means of the closed operating contacts. If the busbar reaches such a high temperature because of too high a current flow in a defect situation that the bimetal switch short-circuits the secondary side of the transformer corresponding thereto, then the voltage drops at the primary side so that the relay opens and the current supply for the busbar is interrupted.
For the known circuit arrangement, the relay circuit serves as an actuator for switching in and out the corresponding actuating quantity, that is, the current supply through the busbar to be monitored. The monitoring arrangement comprises the relay circuit connected to the current source and the bimetal switch parallel to the relay circuit with the corresponding transformer. This monitoring circuit is brought into operative connection with the switching component to be monitored via the bimetal switch. The switching component to be monitored is the busbar.
The use of bimetal switches limits the response precision of the monitoring arrangement because of their switching hysteresis. The large configuration of the switches makes their use in the smallest electronic circuits impossible.
The utilization of thermal fuses is set forth in the German Industrial Standard DIN 57 631 of December 1983 and in the International Standard IEC-691, first edition 1980. The alternative utilization of thermal fuses in electric switching circuits has similar disadvantages, namely: the response times run to several seconds (up to approximately 15 seconds); the response temperature can be selected in only a limited manner in steps; their functional reliability can not be checked with reasonable cost and complexity; and, their switch-off performance is dependent upon their thermal history.
All these disadvantages make the use in fast switching electronic circuits difficult, especially maintaining the reproducible switching temperature for intrinsically safe switching circuits, which are intended to operate in potentially explosive ambient atmospheres, is not guaranteed.